The present invention relates to a method of manufacturing sintered compacts by sintering metal powder, and more particularly to a manufacturing method in which a green body of a predetermined shape containing a metal powder is prepared, and then the green body is subjected to debinding treatment and sintering treatment to produce sintered compacts.
As a process of manufacturing a metal product by sintering a green body containing metal powder, there is known in the conventional art a process named as xe2x80x9cmetal injection molding (MIM)xe2x80x9d. In MIM, metal powder is mixed with an organic binder and then they are compounded to obtain a compound, and then injection molding is carried out using the compound.
A green body prepared by MIM is subjected to a debinding treatment (binder removal treatment) in order to eliminate the organic binder, and then such green body is sintered.
In order to ensure good forming properties during injection molding, a green body used in MIM must contain an organic binder in a fairly large amount. Therefore, the green body which has undergone the debinding treatment (that is, binder removed green body) tends to have a number of pores. When such a binder removed green body having a number of pores is sintered, the following drawbacks will arise.
(1) While density of the sintered compact is lowered, porosity of the sintered compact is high. This results in a sintered compact with low mechanical strength.
(2) Relatively high sintering temperatures are required. Such high temperatures give a large load to the furnace, thus leading to disadvantages that requires expensive equipment and consumes large amounts of power.
(3) It is impossible to obtain high dimensional precision. For example, when a green body has significant variation its thickness, the obtained sintered compact is likely to have a deformed shape.
It is therefore an object of the present invention to provide a method of manufacturing sintered compacts which can obtain sintered compacts having high density, or can obtain sintered compacts having excellent formability, that is, can obtain sintered compacts having high dimensional precision, and which can moderate sintering conditions such as lowering a sintering temperature to be employed or the like.
The present invention is directed to a method of manufacturing a sintered compact, which comprises the steps of:
producing a green body containing metal powder;
debinding the green body at least once;
sintering the debinded green body at least once to obtain a sintered compact; and
compacting the green body by pressing it, wherein the compacting step is carried out at any time after the green body producing step and prior to a completion of the green body sintering step.
By compacting the green body through the pressing, it becomes possible to increase the density of the final sintered compact and to increase the mechanical strength thereof, as well as to improve dimensional precision of the final sintered compact. Therefore, metal products having high quality can be manufactured.
The green body compacting step may be carried out between the green body producing step and the green body debinding step. In this way, even if molding flaws such as pores would be formed during the production of the green body, such flaws are eliminated to bring the green body in good condition. Therefore, when a sintered compact is formed from the green body through the subsequent debinding treatment and sintering, it is possible to obtain a metal product having especially high qualities based on the sintered compact.
In this case, machine working may be performed on the compacted green body before the completion of the green body sintering step, in particular before the start of the debinding treatment. Since the machine working is performed on the green body which has been compacted by pressing, less variations occur in the shape and dimensions at the working area as compared with the case where such machine working would be performed on an uncompacted green body, and therefore it is possible to improve dimensional precision of the sintered compact. In addition, since the machine working is carried out before the completion of the sintering step, hardness of a work is relatively low as compared with the case where such a machine working would be performed on a sintered compact having a high hardness, so that working can be made easily. Further, since workability is also excellent, the shape and dimensions of the working area can be easily controlled, thus leading to improved dimensional precision.
Further, the green body compacting step may be carried out during the debinding step or between the debinding step and the green body sintering step. In this way, it becomes possible to reduce the number of pores present in the green body and thereby to increase its density prior to sintering. This makes it possible to obtain a sintered compact having high density and high mechanical strength as well as to moderate sintering conditions such as lowered sintering temperature, shortened sintering time and the like, thus leading to improved sinterability and reduced load to a sintering furnace and the like.
In this case, machine working may be performed on the compacted green body before the completion of the green body sintering step, in particular before the completion of the debinding treatment or the start of the sintering step. Since the machine working is performed on the green body which has been compacted by pressing, less variations occur in the shape and dimensions at the working area as compared with the case where such machine working would be performed on an uncompacted green body, and therefore it is possible to improve dimensional precision of the sintered compact. In addition, since the machine working is carried out before the completion of the sintering step, hardness of a work is relatively low as compared with the case where such a machine working would be performed on a sintered compact having a high hardness, so that working can be made easily. Further, since workability is also excellent, the shape and dimensions of the working area can be easily controlled, thus leading to improved dimensional precision.
Further, the green body compacting step may be carried out during the green body sintering step. In this way, it is possible to reduce pores present in the green body (presintered compactxe2x80x9d during the sintering process to increase its density, thus enabling to obtain a sintered compact having higher density and higher mechanical strength. Further, it is also possible to moderate sintering conditions such as lowered sintering temperature, shortened sintering time and the like, thus leading to improved sinterability and reduced load to a sintering furnace and the like.
In this case, machine working may be performed on the compacted green body before the completion of the green body sintering step. Since the machine working is performed on the green body (presintered compact) which has been compacted by pressing, less variations occur in the shape and dimensions at the working area as compared with the case where such machine working would be performed on an uncompacted green body (debinded green body or presintered compact), and therefore it is possible to improve dimensional precision of the sintered compact. In addition, since the machine working is carried out before the completion of the sintering step, hardness of a work is relatively low as compared with the case where such a machine working would be performed on a sintered compact having a high hardness, so that working can be made easily. Further, since workability is also excellent, the shape and dimensions of the working area can be easily controlled, thus leading to improved dimensional precision.
Further, in the present invention, it is preferred that the pressing for compaction is carried out isotropically, in particular the pressing for compacting is carried out by means of an isostatic pressing. In this way, it becomes possible to produce a green body and a sintered compact having uniform density with a simple method.
In this case, it is preferred that the isostatic pressing is carried out at ambient temperature or temperature close thereto, because equipment for pressing can be simplified and no heat resistance property is required to waterproof coating film.
In the present invention, it is preferred that the pressing is 1 to 100 t/cm2. This makes it possible to achieve sufficient compaction without requiring large-scale pressing equipment.
Furthermore, in the present invention, it is preferred that the green body producing step is carried out by means of metal injection molding. This makes it possible to manufacture metal sintered products having a relatively small size and/or a complex and intricate shape, and having relatively high mechanical strength.
Moreover, in the present invention, it is also preferred that the metal powder content of the green body Just before the debinding treatment is 70 to 98 wt %. When using such a green body, it becomes possible to ensure good formability when the green body is produced, and to prevent shrinkage from being increased during sintering of the green body.
In the present invention, it is also preferred that the metal powder for the green body is prepared in accordance with a gas atomization method. Particles of meal powder produced by the gas atomization method have a roughly spherical shape, so that it is possible to moderate a particle size of metal powder and pressing conditions. With this result, it becomes possible to enhance the mechanical strength of the obtained sintered compact.
Another aspect of the present invention is directed to a method of manufacturing a sintered compact, which comprises the steps of:
producing a green body containing metal powder;
compacting the green body by pressing it;
debinding the compacted green body at least once; and
sintering the debinded green body at least once to obtain a sintered compact.
Provision of the step for compacting the green body by pressing it makes it possible to increase the density of the final sintered compact and to increase the mechanical strength thereof, as well as to improve dimensional precision of the final sintered compact. Therefore, metal products having high quality can be manufactured. In particular, even if molding flaws such as pores would be formed during the production of the green body, such flaws are eliminated by pressing the green body to bring the green body in good condition. Therefore, when a sintered compact is formed from the green body through the subsequent debinding treatment and sintering, it is possible to obtain a metal product having especially high qualities based on the sintered compacts.
In this case, machine working may be performed on the green body between the green body compacting step and the debinding step. Since the machine working is performed on the green body which has been compacted by pressing, less variations occur in the shape and dimensions at the working area as compared with the case where such machine working would be performed on an uncompacted green body, and therefore it is possible to improve dimensional precision of the sintered compact. In addition, since the machine working is performed on the green body of which hardness is greatly lower than that of the high hardness sintered compact, working can be made easily. Further, since workability is also excellent, the shape and dimensions of the working area can be easily controlled, thus leading to improved dimensional precision.
Other aspect of the present invention is directed to a method of manufacturing a sintered compact, which comprises the steps of:
producing a green body containing metal powder;
conducting a first debinding treatment on the green body;
compacting the debinded green body by pressing it;
conducting a second debinding treatment on the compacted green body; and
sintering the debinded green body at least once to obtain a sintered compact.
Provision of the step for compacting the green body by pressing it makes it possible to increase the density of the final sintered compact and to increase the mechanical strength thereof, as well as to improve dimensional precision of the final sintered compact. Therefore, metal products having high quality can be manufactured. In particular, it is possible to reduce the number of pores present in the green body and thereby to increase its density prior to sintering. This makes it possible to obtain a sintered compact having high density and high mechanical strength as well as to moderate sintering conditions such as lowered sintering temperature, shortened sintering time and the like, thus leading to improved sinterability and reduced load to a sintering furnace and the like.
In this case, machine working may be performed on the green body between the green body compacting step and the step for conducing the second debinding treatment. Since the machine working is performed on the green body which has been compacted by pressing, less variations occur in the shape and dimensions at the working area as compared with the case where such machine working would be performed on an uncompacted green body, and therefore it is possible to improve dimensional precision of the sintered compact. In addition, since the machine working is carried out before the completion of the sintering step, hardness of a work is relatively low as compared with the case where such a machine working would be performed on a sintered compact having a high hardness, so that working can be made easily. Further, since workability is also excellent, the shape and dimensions of the working area can be easily controlled, thus leading to improved dimensional precision.
The other aspect of the present invention is directed to a method of manufacturing a sintered compact, which comprises the steps of:
producing a green body containing metal powder;
debinding the green body at least once;
presintering the debinded green body;
compacting the presintered green body by pressing it; and
sintering the compacted presintered green body further to obtain a sintered compact.
Provision of the step for compacting the green body by pressing it makes it possible to reduce pores present in the presintered compact and to increase the density thereof, thus enabling to obtain a sintered compact having a higher density and a higher mechanical strength. Further, this also makes it possible to moderate sintering conditions such as lowered sintering temperature, shortened sintering time and the like, thus leading to improved sinterability and reduced load to a sintering furnace and the like.
In this case, machine working may be performed on the compacted presintered green body between the presintered green body compacting step and the presintered green body sintering step. Since the machine working is performed on the presintered compact which has been compacted by pressing it, less variations occur in the shape and dimensions at the working area as compared with the case where such machine working would be performed on an uncompacted green body (debinded green body or presintered compact), and therefore it is possible to improve dimensional precision of the sintered compact. In addition, since the machine working is carried out before the completion of the sintering step, that is prior to the main sintering process, hardness of a work is relatively low as compared with the case where such a machine working would be performed on a final sintered compact having high hardness, so that working can be made easily. Further, since workability is also excellent, the shape and dimensions of the working area can be easily controlled, thus leading to improved dimensional precision.
In this case, it is preferred that the green body presintering step is being carried out until diffusion bonding is made at least at contact points of particles of the metal powder. Conducting presintering in this way increases the shape stability. As a result, it becomes possible to reliably prevent various flaws of the green body (presintered compact) such as breaking, chipping, cracking and the like from occurring during the subsequent compacting step and the machine working process, thus improving handling ability thereof.